Sensors for the measurement of various physical parameters such as pressure and temperature often rely on the transmission of strain from an elastic structure (e.g., a diaphragm, bellows, etc.) to a sensing element. In a pressure sensor, the sensing element may be bonded to the elastic structure with a suitable adhesive.
It is also known that the attachment of the sensing element to the elastic structure can be a large source of error if the attachment is not highly stable. In the case of sensors which measure static or very slowly changing parameters, the long term stability of the attachment to the structure is extremely important. A major source of such long term sensor instability is a phenomenon known as "creep", i.e., change in strain on the sensing element with no change in applied load on the elastic structure, which results in a DC shift or drift error in the sensor signal.
Certain types of fiber optic sensors for measuring static and/or quasi-static parameters require a highly stable, very low creep attachment of the optical fiber to the elastic structure. One example of a fiber optic based sensor is that described in U.S. Pat. No. 6,016,702 entitled "High Sensitivity Fiber Optic Pressure Sensor for Use in Harsh Environments" to Robert J. Maron, which is incorporated herein by reference in its entirety. In that case, an optical fiber is attached to a compressible bellows at one location along the fiber and to a rigid structure (or housing) at a second location along the fiber with a Bragg grating embedded within the fiber between these two fiber attachment locations. As the bellows is compressed due to an external pressure change, the strain on the fiber grating changes, which changes the wavelength of light reflected by the grating. If the attachment of the fiber to the structure is not stable, the fiber may move (or creep) relative to the structure it is attached to, and the aforementioned measurement inaccuracies occur.
One common technique for attaching the optical fiber to a structure is epoxy adhesives. It is common to restrict the use of epoxy adhesives to temperatures below the glass transition temperature of the epoxy. Above the glass transition temperature, the epoxy transitions to a soft state in which creep becomes significant and, thus, the epoxy becomes unusable for attachment of a sensing element in a precision transducer. Also, even below the glass transition temperature significant creep may occur.
Another technique is to solder the structure to a metal-coated fiber. However, it is known that solders are susceptible to creep under certain conditions. In particular, some soft solders, such as common lead-tin (PbSn) solder, have a relatively low melting point temperature and are thus relatively unsuitable for use in transducers that are used at elevated temperatures and/or at high levels of stress in the solder attachment. The use of "hard" solders with higher melting temperatures, such as gold-germanium (AuGe) and gold-silicon (AuSi), can reduce the problem; however, at elevated temperatures and/or high stress at the solder attachment, these hard solders also exhibit creep. In addition, the high melting temperature of such solders may damage the metal coating and/or damage the bond between the metal coating and glass fiber.